Electrical apparatus



Jan. 31, 1967 o, 5050 ET AL 3,302,097

ELECTRICAL APPARATUS Original Filed Nov. 9. 1962 6 Sheets-Sheet 1 Fig. l.

INVENTORS Powell 0. Bobo and Frederick W. Keoy.

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To 68 REGULATION ggLll CIRCUITS United States Patent 3,3{i2,097 ELECTRICAL APPARATUS Powell (I. Bobo, Pittsburgh, and Frederick W. Keay, Irwin, Pa, assignors, by mesne assignments, to the United States of America, as represented by the United States Atomic Energy Commission Continuation of appiication Ser. No. 236,529, Nov. 9 1962. This application Mar. 7,, 1966, er. No. 541,879

2 Claims. (@Cl. 322-25) ently produces excellent transient response because field' forcing is instantaneously avaliable. Also, this method of excitation provides good voltage regulation because the system supplies nearly the correct excitation for any load and power factor.

When more accurate voltage regulation is required, a voltage regulator may be added to the excitation system. Most of the regulated systems employ continuous acting regulators which function through one or more stages of amplification to control all or part of the current and potential derived energy. The control is usually accomplished by controlling the saturation of a transformer element in the system by means of a unidirectional signal obtained from the regulator and associated amplifier circuitry. These regulating systems, however,v have disadvantages in that the transient response of the system is limited by the time delay characteristics of the control elements, which can be appreciable, particularly in the larger power units. Also, the large power control elements required are bulky, expensive, and require appreciable control energy from the regulator and amplifier circuit.

Further, prior art systems have certain disadvantages when a specified fault cur-rent is required to be maintained for purposes-of selective breaker tripping. This is usually accomplished by saturation of current transformers or inductive reactors. Saturation of current transformers is undesirable because high voltages are produced which can damage other circuit components. Reactor control is also undesirable because of the large amount of control power required and the difiiculty in obtaining the proper control signal for three phase and single phase faults. It is therefore desirable that a static excitation system for self-excited alternating-current generators be provided that has excellent transient response, has only a small portion of the excitation current controlled by the regulating element, and uses linear, standard components.

Accordingly, it is the general object of this invention to provide a new and improved excitation system for a dynamoelectric machine.

Another object of this invention is to provide a new and improved regulating system for controlling the excitation of a dynamoelectric machine.

It is a more particular object of this invention to provide a new and improved .static excitation system that has improved transient response characteristics for a dynamoelectric machine.

It is a further object of this invention to provide a new and improved method of limiting fault current in a dynamoelect-ri-c machine.

Briefly, the present invention accomplishes the above cited objects by supplying more current to the generator field circuit than is required at any given operating load and power factor. Regulation is then accomplished by shunting the excess current away from the generator field by means of suitable variable shunt circuits responsive to an error signal produced by a regulator proportional to the deviation of the generator output voltage from a reference voltage. The excitation current applied to the generator field circuit is responsive to both the output voltage and current of a dynamoelectric machine and is compensated for changes in power factor. This current is applied directly to the filed circuit of the dynamoelectric machine without going through any control devices. Only a very small error current goes through the control devices. Therefore the control devices may be small, inexpensive, and easily standardized to cover a wide range of generator ratings. This arrangement provides an excitation system having the ultimate in response, since field forcing is instantaneously available. Transient response is oniy limited by the characteristics of the machine itself.

The invention also allows a specified fault current to be maintain-ed without the disadvantages hereinbefore described. Fault current may be limited to any desired value by the shunt control in response to either voltage or current sensitive static devices.

Further objects and advantages of the invention will become apparent as the following description proceeds and features of novelty which characterize the invention will be pointed out in particularity in the claims annexed to and forming a part of this specification.

For a better understanding of the invention, reference may be had to the accompanying drawings, in which:

FIGURE 1 shows the basic electrical circuit embodying thet teachings of this invention;

FIGS. 2, 3 and 4 are vector diagrams explanatory of the operation of a portion of the circuit shown in FIG. 1;

FIG. 5 is a schematic diagram of a voltage regulator that may be used in the circuit shown in FIG. 1;

FIG. 6 is a schematic diagram of a current limiting circuit that may be used in the circuit shown in FIG. 1;

FIG. 7 is a schematic diagram of a pro-amplifier and bias circuit that may be used in the circuit shown in FIG. 1;

FIG. 8 is a schematic diagram of a firing control circuit for controlled recti-fiers that may be used in the circuit shown in FIG. 1;

FIG. 9 is a schematic diagram illustrating another embodiment of the invention; and

FIG. 10 is a schematic diagram illustrating still another embodiment of the invention.

FIGURE 1 shows a circuit diagram of a basic circuit r embodying the teachings of this invention. It is a com pletely self-excited system, with the total excitation current being derived from the output voltage of the dynamoelectric machine, specifically the three-phase alternatingcurrent generator 12. In the embodiment shown in FIG. 1, the generator 12 comprises an armature 14 and an excitation field winding 16. The generator 12 is disposed to supply alternating-current energy to line conductors 18, Zii and 22 through output terminals 24, 26 and 28, respectively, of armature 14.

In general, current greater than the excitation requirements of the alternating current generator is produced by vector summing circuit 40 and applied to the field rectifier circuit 36. Current produced by the vector summing circuit 40 could be arranged to provide close to the proper excitation required by the generator, however, to compensate for voltage output changes, the vector summing circuit 40 is arranged to provide slightly more current than the value of excitation current I required by the generator 12 at any given load and power factor over the operating range. The excess excitation current is shunted away from the field 16 in response to the signal from the voltage regulator 50, as amplified by preamplifier 60. The shunting of a certain portion of the current produced by vector summing circuit 40 accomplishes the required voltage regulation because the amount of current shunted is proportional to an error signal or signal proportional to the deviation of the generator output voltage from a predetermined value. It will be noted that the field excitation current, 1,, is impressed directly upon the generator field 16 by virtue of rectifier 36, without going through any control device. Therefore, the current from vector summing circuit 40 is not delayed from reaching the generator field 16 and any change in said vector summing current is immediately reflected in the field circuit currentI Current I; is proportional to the vector sum of two currents produced in each phase of vector summing circuit 40. One of these currents, I is proportional to the generator line current I and is produced by series connected primary windings 46, 48 and 52 of vector summing transformers 62, 64 and 66. The other current I is produced by shunt connected windings 68, 70 and 72 of vector summing transformers 62, 64 and 66 and is responsive to the generator line-to-neutral voltage, but lagging it by substantially 90 by action of reactors 74, 76 and 73. The reactance of reactors 74, 76 and 78 is large compared with the circuit resistance. Thus, the current I is proportional to the generator output voltage but substantially 90 out of phase in the lagging direction with it. As hereinbefore stated, the currents I and I combine vectorially in each phase to form currents I I and I which are applied to rectifier 36 at terminals 82, 84 and 86. FIGURES 2, 3, and 4 show vectorially the combining of currents I and I for different circuit power factor conditions. FIGURE 2 illustrates the formation of current from one of the phases, such as current I formed from current I and 1 at unity power factor. Current I is in phase with the line-to-neutral voltage V and current 1 is out of phase with voltage V by nearly 90 because of the inductive reactance of reactor 74. The resultant current I is formed as illustrated in FIG. 2.

FIGURE 3 illustrates the formation of current I from currents I and I at zero power factor lag. As illustrated, both currents I and I lag the line-to-neutral voltage V by approximately 90 and the resultant current I is equal, in this instance, to the arithmetic summation of the currents I and I FIGURE 4 illustrates the formation of current I from currents I and I at an intermediate lagging power factor. Current I lags the line-to-neutral voltage V by some angle between Zero and 90 and current I lags line-toneutral voltage V by approximately 90. In this instance, the resultant current I is the vector sum of currents I and I FIGURES 2, 3 and 4 graphically illustrate that the invention will remain in control and supply the excitation requirements at any circuit power factor. As the circuit power factor changes, the resultant current I changes accordingly to compensate for the power factor change.

More specifically, one side of inductive reactor 74 is connected to line conductor 18 at terminal 88. The other side of inductive reactor 74 is connected to one side of primary winding 68 of vector summing transformer 62. Similarly, reactor 76 has one side connected to line conductor 20 at terminal 90 and the remaining side of reactor 74 is connected to one side of primary winding 70 of vector summing transformer 64. Reactor 78 has one side connected to line 22 at terminal 92 and its remaining side is connected to one side of primary winding 72 ofvector summing transformer 66. The remaining sides of primary windings 68, 70 and 72 of vector summing transformer 62, 64 and 66 each have one side connected together at a common point 101. The primary winding 46 of current transformer 62 is connected in series with main conductor 18; the primary winding 48 of vector summing transformer 64 is connected in series with main conductor 20; and primary winding 52 of vector summing transformer 66 is connected in series with main conductor 22. The secondary windings 102, 104 and 106 of vector summing transformers 62, 64 and 66, have sides 116, 118 and 120 connected together and the common point connected to terminal 115 of reset and firing circuit 80. Side 122 of secondary winding 102 of vector summing transformer 62 is connected to rectifier 36 at terminal 86 and rectifier 120 at terminal 112. Similarly, side 124 of secondary winding 104 of vector summing transformer 64 is connected to rectifier 36 at terminal 84 and rectifier 120 at terminal 110. The remaining vector summing transformer 66 has side 126 of secondary winding -106 connected to terminal 82 of rectifier36 and terminal 108 of rectifier 120.

The vector summing transformers 62, 64 and 66 and reactors 74, 76 and 78 are selected sothat the combination of series and shunt derived components of excitation current provides slightly more current than the value of excitation current I required by the generator 12 at any given load and power factor. Also, as hereinbefore stated, the currents I I and I are applied directly to the field winding 16 through rectifier 36. Current I, does not go through any control devices which tend to give poor transient response.

As previously described, the components of vector sun ming circuit are chosen so that they supply a greater current than the current I required by the generator 12 at any given load and power factor. The portion of the current to be shunted away from the field winding 16 and, therefore,'produce the proper field excitation current I, is determined by the regulator in combination with current limiting circuit 55. The regulator 50 senses any deviation of the output voltage from the generator 12 from a preset value. This is accomplished by lines 122, 124 and 126 of the regulator 50 to terminals 112, 114 and 116 on line conductors 22, 20 and 18, respectively. If desired, a current limiting circuit may be provided which sensesthe generator line current I through current transformers 71, 73 and 75. Current limiting circuit 55 operates in conjunction with the voltage regulator 50 to supply a signal to the preamplifier andbias circuit when the output of generator 12 is shorted. Terminal 63 of current limiting circuit 55 is connected to one side of current transformer 71. Terminal 'of current limiting circuit 55 is connected to .terminal 69 of voltage regulating circuit 50. Terminal67 .of current limiting circuit 55 is connected to one side of current transformer 75. One side of current transformer 73 is connected to voltage regulator circuit 50 at terminal 91. The remaining sides of current transformers 71, 73 and are connected together and grounded at point 77. One side of the voltage regulator output goes through the current limiting circuit, with terminal 61 of voltage regulating circuit 50 connected to terminal 59 of current limiting circuit 55. Terminal 57 of current limiting circuit 55 is connected through line 132 to preamplifier and bias circuit 60. Any deviation of the output voltage on the conductors 18, 20 and 22 from generator 12 is turned int-o an error signal by regulator 50 proportional to the amount of this deviation. The error signal produced by regulator 50 is applied to terminals 144 and 146 of preamplifier and bias circuit 60 through lines 132 and 134. The peamplifier and bias circuit 60 may obtain its power from the generator output through a separate transformer, or, as shown, it may obtain its output fromthe secondary of vector summing circuit 40. In this instance, terminals and 142 of preamplifier and bias circuit 60 are connected through lines 136 and 138 to lines 148 and 150 from vector summing circuit 40. The amplified error signal and bias voltages from terminals 148, 150, 152 and 154 from preamplifier and bias circuit are applied to terminals 156, 158, 160 and 162 of reset and firing circuit 80. A signal proportional to the error signal from the voltage regulator 50 is applied to rectifier circuit 120 from terminals 164, 166, 168 and 169 through lines 170, 172, 174, and 175', respectively. In order to obtain electrical signals synchronized with the output of each phase of vector summing circuit 40, terminals 109, 111, 113 and 115 of reset and firing circuit are connected to synchronizing pulse transformers 183, 185, and 187, and said pulse transformers are disposed in inductive relationship with lines 152, 150 and 148 respectively from the secondary of vector summing circuit 40. More specifically, pulse transformer 183 has one side connected to terminal 109, pulse transformer 185 has one side connected to terminal 111, and pulse transformer 187 has one side connected to terminal 113. The remaining sides of pulse transformers 183, 185 and 187 are connected to terminal and terminal 115 is also connected to the common point of the secondary windings 102, 104 and 106 of vector summing transformer 62, 64 and 66. Synchronizing pulse transformers 183, 185 and 187 saturate very early in the cycle and put out a spike each time their ampere turns pass through zero.

The rectifiers 36 and form the static excitation circuit for supplying the correct field excitation current I to field winding 16 of generator 12. The rectifier 36 is a three phase, full wave, dry type rectifier which may be comprised of semiconductor diodes 181 in each leg of the rectifier. The rectifier 120 is also a three phase, full wave, dry type rectifier which also may be comprised of semiconductor diodes 181. However, rectifier 120 has a switching device in one leg of each phase, which may be semiconductor controlled rectifiers 176 and 177, and 178. Each semiconductor controlled rectifier has a gate or control electrode G, an anode A and a cathode C. The output of rectifier 120 is connected through lines 180 and 182 to resistor 184 The lines 170, 172 and 174 from the reset and firing circuit 80 to rectifier 120 previously referred to, are connected to the gate or control electrodes G of controlled rectifiers 174, 176 and 178 respectively. Line is connected to rectifier 120 at line 180.

In the operation of the static excitation system shown in FIGURE 1, the reactors 74, 76 and 78 and shunt connected primary windings 68, 70 and 72 of vector summing transformers 62, 64 and 66 respectively, produce a current proportional to the voltage output of the generator 12 but substantially 90 out of phase in the lagging direction with the generator line-t-o-neutral voltage because of the inductive action of said reactors. Another component of current proportion-a1 to the generator output current is produced in series connected primary windings 46, 48 and 52 of vector summing transformers 62, 64 and 66. The current proportional to generator ,voltage and the current proportional to generator output current from generator armature 14 combine vectorially in said vector summing transformers and produce an output current in the vector summing transformer secondarie proportional to the vector sum of the two currents in the primary windings. Secondary 102 of vector summing transformer 62 produces a current I proportional to the vector sum of currents I and I Similarly, secondary winding 104 of vector summing transformer 64 produces a current l and secondary winding 106 of vector summing transformer 66 produces a current 1,. The currents I I and I are applied to the 3 phase, full wave rectifier 36 and they are rectified by the rectifier diodes 181, and a direct current voltage is applied to the busses 30 and 32 which supply the filed winding 16 with excitation current. The direct current I produced by rectifying the vector summing currents I I and I is slightly in excess of the field current required by the field winding 16 under any condition of load and circuit power factor. Voltage regulation is then accomplished by shunting of the excess excitation current away from the field windings. In this instance, the shunting of the excess current is accomplished by means of controlled rectifiers 176, 177 and 178 in the rectifier 120. Since the output of vector summing circuit 40 is connected to rectifier 120 as well as rectifier 36, when the controlled rectifiers conduct, the resistor 184 is connected across the secondary of the vector summing transformer 62, 64 and 66 similar to the field winding 16 which is also connected across the secondaries of vector summing transformers 62, 64 and 66. When the controlled rectifiers conduct and connect the resistor 184 in parallel with the field winding 16, a portion of the current from vector summing circuit 40 is shunted through the resistor 184. The amount of current shunted through resistor 184 and away from field winding 16 is controlled by varying the firing angle of the controlled rectifier in proportion to the amplified error signal derived from the voltage regulator and current limiting circuits 50 and 55, the preamplifier and bias circuit 60 and firing circuit 80. The controlled rectifiers do not conduct until a signal is introduced or applied to the gate electrode G. Once conduction has been initiated, the gate electrode G loses control and the controlled rectifier continues to conduct until the current through the anode-cathode circuit falls below a small value commonly called the holding current. Therefore, each time the current drops to zero, the gate regains control and conduction is stopped until another signal is applied to the gate electrode G. By varying the point during the positive cycle that the controlled rectifiers conduct, the time that the current is being shunted through the resistor 184 can be varied. Therefore, an average field current If can be established to provide the proper excitation for the generator 12 under all conditions of load and power factor.

It should be noted that this arrangement provides a fail safe excitation system in that a failure of the regulator and/ or amplifier will result in only a slight change in the generator output voltage, as the current shunted by the regulator-amplifier combination is a (very small portion of the total excitation current 1,. Further, the invention does not require the use of any non-linear components. Therefore, accurate calculations can be made predicting the performance of the system. Also, no frequency sensitive components are required, eliminating the requirement of any complicated frequency compensation networks. Other advantages of supplying the major excitation current in self-excited alternating-current generators without going through control devices result from the fact that the control components that are used are low power devices and may be standardized or identical for all applications over a large range of alternating-current generator sizes.

In order to insure positive build-up of the alternatingcurrent generator voltages at terminals 24, 26, and 28, a separate source of power may be provided for momentary field flashing. This source of power may be a battery 13 or a source of alternating current rectified by separate rectifiers and applied to the generator field winding 16. The momentary application of direct current power to the field circuit may be accomplished during start up by means of contacts 15 on a spring return start-up switch.

A voltage regulator 50 that may be used is shown in FIG. 5. In general, voltage regulator 50 is comprised of a reactive curent compensating circuit 200 a three phase full wave rectifier 202 and error detecting bridge 204.

The compensator circuit 200 comprises transformers 204, 206 and 208, reactor 210, and resistor 212. The action of this circuit is such that with normaL-voltage output of the generator, the alternating current voltage applied to the rectifier 202 in the regulator 50 is increased above the normal value in direct proportion to the reactive current being supplied by the generator. An increase in reactive current, therefore, causes the voltage regulator 50 to put out a signal to lower the generator output volt- 7 age. Thus, the generator output voltage is caused to drop in proportion to the reactive current being delivered by the generator. This action of the compensator circuit 200 enables alternating current generators operated in parallel to share the reactive load in proportion to their ratings.

The reactive current compensating circuit 200 has its input terminals 91 and 69 connected to current tr-ansformer 73 and current limiting circuit 55 as shown in FIG. 6. Input terminals 69 and 91 are connected to tapped transformer 204. The output of transformer 204 is mixed with the voltage signal in current transformers 206 and 208 and applied to rectifier 202. The inductor 210 and variable resistor 12 provide a balance adjustment for the output of transformer 204. Input terminals 83, 85 and 87 of voltage regulator circuit 280 are connected as shown in FIG. 5, to potential transformer 214 and the potential transformer 214 is in turn connected to current transformers 206 and 208 and rectifier 202. The input signal to input terminals 83, 85 and 87 of voltage regulator 50 is thus responsive to and varies with the output voltage of the generator 12. The voltage signal obtained, as hereinbefore stated, is modified by the reactive current compensating circuit from compensating circuit 200 in current transformers 206 and 208. The modified signal is applied to the three phase, full wave rectifier 202 which may be comprised of semiconductor diodes 216, and the rectified signal is applied to error detecting bridge circuit 203. The settings of the variable resistor 218 and adjustable rheostat 220 determine the output voltage to be maintained by the regulator.

The error detecting bridge 203 may be a conventional error detecting circuit comprising Zener diodes 222 and 224 in two legs of the bridge and resistors 230 and 232 in the remaining bridge legs. Any deviation between the magnitude of the reverse break-down voltage of the Zener diodes 222 and 224 and the generator output volt age signal appears at terminals 234 and 238 of error detecting bridge 204 as a polarized direct current error signal. More specifically, when a direct current voltage is applied to the input terminals 240 and 242 which is equal to twice the voltage drop across each Zener diode 222 and 224, there will be substantially no voltage difference at the output terminals 234 and 238. When the input voltage is above or below the reference voltage of the bridge, then a voltage difference will exist across the output terminals 234 and 238. The polarity of this signal depends upon whether the input signal is above, or below, the desired voltage as determined by the setting on generator voltage adjustment 220. The error signal from voltage regulator 50 is applied to output terminals 61 and 93; output terminal 61 is connected to terminal 59 on current limiting circuit 55 and output terminal 93 is connected to terminal 146 on preamplifier and bias circuit 60.

A current limiting circuit that may be used is shown in FIG. 6. When the generator output is shorted, the voltage falls to zero and hence the voltage regulator no longer puts out the desired signal. The function of the current limiting circuit 55 is to provide a signal to the preamplifier and bias circuit during the short circuit condition to limit the short circuit current. This is accomplished, in this instance, by obtaining a voltage proportional to the generator output current through current transformers 71, 73 and 75 shown in FIG. 1, rectifying the signal in dry type rectifier 260 shown in FIG. 6 and applying it to Zener diode 262. If the generator output is within predetermined limits, the voltage developed will not be sufiicient to exceed the reverse breakdown voltage of the Zener diode 262, and hence the current limiting circuit 55 will not provide a signal during this period. If the generator current exceeds a predetermined value, a voltage sufficient to cause the Zener diode to conduct is produced, and a signal is applied to the preamplifier and bias circuit 60. This signal, when amplified and applied to the firing circuit and controlled rectifiers 176, 177 and 178, will limit the generator output current. The current transformers 264, 266 and 268, which have one side of their primary windings 270, 272 and 274 connected to terminals 63, 65 and 67 and the other side of their primaries connected together at a common point 281, are auxiliary transformers for obtaining a suitable range of voltage for proper operation of the circuit components.

The secondary windings 276, 278 and 280 of current transformers 264, 266 and 268 each have one side connected to a leg of rectifier 260 at junctions 284, 286 and 288 and the remaining sides are connected together at a common point 282. The rectifier 260 is a three phase, full wave rectifier and may be comprised of semiconductor diodes 290.

Adjustable resistor 292 connected across the output terminal 302 and 304 of rectifier 260 allows selection of the generator current magnitude that will cause current limiting circuit 55 to produce a signal.

Rectifier 294, which may be a semiconductor diode is connected between the Zener diode 262 and terminal 57 to block the signal from the voltage regulator 50. Resistor 296 is connected between terminals 57 and 59 and carries the signal from the voltage regulator 50 from terminal 59 to terminal 57 so that it can continue on to the preamplifier and bias circuit 60.

Although a circuit sensitive to generator output current is shown in FIG. 6, it is to be understood that a circuit sensitive to generator field voltage could also be used to obtain a signal that would limit current by the generator field shunting method.

A preamplifier and bias circuit 60 may be used as shown in FIG. 7. The bias circuit 310 has its input terminals and 142 connected to the secondary windings of the vector summing circuit 40, as shown in FIG. 1. Input terminals 140 and 142 are connected to the primary winding 316 of transformer 320 through resistor 322. Rectifiers 324 and 326, which may be Zener diodes, limit the incoming voltage to a predetermined value. The secondary windings 328 and 330 of transformer 320 are connected to single phase, full wave dry type rectifiers 312 and 314. Secondary winding 328 is connected to the input terminals 332 and 334 of rectifier 312 and secondary winding 330 is connected to input terminals 336 and 338 or rectizer 314. Rectifiers 312 and 315 may be comprised of semiconductor diodes 313. The ripple in the direct current voltage ouput of rectifiers 312 and 314 may be filtered by inductor 348 and capacitor 350, and inductor 352 and capacitor 354, respectively. Resistors 356 and 358 are ballast resistors for the Zener diodes 360 and 362. Zener diodes 360 and 362 provide a regulated direct current voltage at output terminals 148, and 152 by conducting current in their reverse direction when the desired voltage is exceeded. Bias voltage for the preamplifier circuit 320 is obtained from bias circuit 310 and further regulated by a Zener diode 364 connected across the bias supply voltage to the preamplifier circuit 320 at junctions 368 and 370. Resistor 366 is a ballast resistor for the Zener diode 364.

The preamplifier circuit 320 is a low-level direct current amplifier whose purpose is to amplify the voltage level of the error signals from the voltage regulator 50 to a magnitude required by the firing circuit 80. The additional gain provided by the preamplifier circuit 320 is required to give the regulating system the desired accuracy.

The preamplifier in general consists of a chopper circuit 400, an alternating current amplifier 410, a synchronous switch 420 and an oscillator circuit 430.

The input terminals 144 and 146 of the preamplifier circuit 320 are connected to the chopper circuit 400 at junctions 432 and 434. The semiconductor rectifiers 436 and 438 across the input terminals 144 and 146 limit the signal input to a voltage below the forward breakdown voltage of the rectifiers and thus protects the tran sistors used in the chopper circuit. The chopper circuit includes transistors 440, 442, 444 and 446, resistors 448, 450, 452 and 454, and windings 456, 458, 460 and 462 of transformer 470.

As will be described in greater detail later, the voltage across windings 456, 458, 460 and 462 of transformer 470 is a square wave. The polarity of the windings is such that transistors 440 and 446 and 442 and 444 are saturated on alternate half cycles. When transistors 440 and 446 are saturated, current flows through these transistors. On the alternate half cycle, the current flows through transistors 442 and 444. The direct current voltage applied to the chopper circuit 400 thus is converted to a square wave or an alternating voltage and is applied to the primary 482 of transformer 480. The voltage appearing at the secondary 484 of transformer 480 is amplified in amplifier 410, which is a typical class B, push-pull amplifier. The amplifier 410 includes transistors 486 and 488, and resistors 490 and 492. Resistors 490 and 492, in series with the emitters of transistors 486 and 488, stabilize said transistors so that they will not go into thermal runaway at elevated junction temperatures. Bias voltage for transistors 486 and 488 is obtained across resistor 494 and resistor 496 from the bias circuit 310. The output of the amplifier 410 is applied to primary winding 498 of transformer 500. The secondary 502 of transformer 500 is connected to the synchronous switch circuit 420. The synchronous switch circuit 420 includes transistors 504, 506, 508 and 510, resistors 512, 514, 516 and 518 and windings 520, 522, 524 and 526 of transformer 470. The synchronous switch is electrically similar to the chopper circuit 400, but performs the opposite function. The chopper circuit 400 has the function of converting a direct current voltage into an alternating current which can be transformed and applied to the alternating current amplifier. Conversely, the synchronous switch takes the alternating current voltage applied to it by the alternating current amplifier 410 and rectifies it to a direct current voltage. Since the windings of transformer 470 that drive the chopper circuit 400 and synchronous switch circuit 420 are common to the same core 471, they operate at the same frequency and phase relationship. The ripple in the direct current output from the synchronous switch circuit 420 is filtered by inductor 530 and capacitor 528 and applied to the output terminals 154 and 150.

The oscillator 430 includes transistors 532 and 534, each having an emitter electrode 2, collector electrode c, and base electrode b, and windings 536, 538, 540 and 542 of transformer 470. The core 471 of transformer 470 is constructed of square loop material. To understand the operation of oscillator 430, consider resistor 544 an open circuit, consider transistor 534 just switched on, or saturated and transistor 532 just cut-off or non-conducting. The supply voltage appears across winding 540 of transformer 470. A proportional voltage is induced into the other windings of transformer 470. The voltage across winding 542 keeps transistor 534 saturated and the voltage across winding 536 keeps transistor 532 out off or non-conducting. As the current continues to increase,

the arrangement of the windings 536, 538, 540 and 542 causes transistor 534 to come out of saturation. When transistor 534 comes out of saturation, the flux in the core 471 of transformer 470 reverses, reversing the polarity on winding 536. The flux reversal starts transistor 532 into saturation and transistor 534 is then'cut off or made non-conducting. This process continues, producing square wave voltages in all windings, as long as direct current voltage is applied to the oscillator circuit 430.

Should transistors 532 and 534 become non-conducting at the same time, the circuit will not go into oscillation because only leakage currents will flow through the transistors 532 and 534 and there will be no flux reversal in the core 471 of transformer 470. This condition could arise when voltage is initially applied to the circuit. To insure that the circuit will go into oscillation, resistor 544 is connected between the collector electrode c of transistor 532 and the base electrode b of transistor 534. When voltage is first applied, current fiows from emitter electrode e to base electrode b of transistor 534 through resistor 544 and winding 538. This current fiow saturates transistor 534 and starts oscillation.

A reset and firing circuit that may be used, and is the subject of copending application Serial No. 236,521 filed November 9, 1962 by H. A. Steinbruegge et a1. and assigned to the same assignee as the present application, is illustrated in FIG. 8. As hereinbefore described and shown in FIG. 1, controlled rectifiers 176, 177 and 178 are used as static switches to shunt current away from the field winding 16 of the generator 12 in response to an error signal. The controlled rectifiers 176, 177 and 178 are switched on once during each positive cycle and shunt a portion of the current from vector summing circuit 40 through resistor 184 until the end of the cycle. The earlier in the cycle the controlled rectifiers are switched, the greater the reduction in field current I The purpose of the firing circuits is to supply to the gate electrodes g of the controlled rectifiers 176, 177 and 178 variable width pulses whose width is determined by the error signal produced by the voltage regulator 50 and current limiting circuit 55. These pulses always start after the beginning of the positive half cycle and end at the termination of the positive half cycle. The delay angle at which the pulses start after the beginning of the positive half cycle is determined by the error signal. Firing circuit produces variable width voltage pulses instead of voltage spikes to fire the controlled rectifiers 176, 177 and 178, to positively ensure firing of the controlled rectifiers and consequently produce stable operation. With an inductive load, as hereinbefore described, instabilities could occur if voltage spikes were used as the firing means, as the spikes may occur while the inductance is discharging and hence the controlled rectifiers may fail to break down and conduct. To accurately control the firing of the controlled rectifiers 1'76, 177 and 178, the variable width pulses produced by the reset and firing circuit 80 must be synchronized with the output of the vector summing transformer and the delay angle must be controlled as a function of the error signal. A method of accomplishing the synchronizing comprises connecting a saturating transformer into the secondaries of each of the vector summing transformers 62, 64 and 66 as shown in FIG. 1. The saturating transformers saturate early in the voltage cycle and put out a voltage spike each time their ampere turns pass through zero. These spikes are used for synchronizing purposes.

The reset and firing circuit shown in FIG. 8 utilizes RC or resistance-capacitance networks 600 to filter the pulses from the synchronizing pulse transformers 183, 185 and 187 shown in FIG. 1. These pulses are used to control solid state flip-flop circuits 610 which put out square waves for synchronizing and reset purposes.

During one half cycle of the reset square wave from synchronous square wave generator circuit 610, a condenser 612 in ramp generator circuit 620 is allowed to charge at substantially a linear rate. The linear voltage increase, having in general a saw tooth shape, is referred to as the ramp voltage. This ramp voltage is compared with the error voltage applied to terminal 162 in differential amplifier circuit 636. As soon as the increasing ramp voltage from ramp generator circuit 620 exceeds the error voltage, the differential amplifier voltage increases and is quickly driven to ceiling. This rapid change in the differential amplifier output voltage is used to turn on flip-flop circuit 640 producing a steep front variable width pulse which is amplified in amplifier 650 and applied to terminal 164 and hence to the gate electrode G of one of the controlled rectifiers. This signal is held constant until the end of the positive half cycle. At

this point, the reset voltage turns on and turns off the flip-flop circuit 640 and discharges the condenser 612. The condenser 612 is held at zero potential for a half cycle until the reset voltage terminates and the cycle is then repeated. Reducing the magnitude of the error signal allows the differential amplifier 630 to increase its output earlier in the cycle, i.e., the ramp voltage exceeds the voltage earlier in the cycle thus reducing the delay angle, and therefore shunting more current away from the field winding 16. Increasing the error signal causes the differential amplifier 630 to increase its output latter in the cycle, thus increasing the delay angle and therefore shunting less current away from the field winding 16.

In this instance we have three firing circuits because we have three controlled rectifiers 176, 177 and 178. However, if six controlled rectifiers were used, as would be required where the secondaries of the vector summing transformers are connected in a six phase star and rectified in a six phase half wave rectifier, only three synchronizing circuits as shown in FIG. 8, would be required. The voltages on the two synchronizing flip-flop output terminals are 180 out of phase so that while one flip-flop output is resetting one condenser, the other output is allowing the diametrically opposite condenser to charge. Therefore, in this instance where only three controlled rectifiers are used, only output terminals 622, 626 and 632 are utilized and terminals 624, 628 and 634 are not used in this instance.

If the controlled rectifiers are fired at zero delay, the maximum current is shunted away from the field winding 16. As the delay angle approaches 180, the shunted current approaches zero. The current through the shunting resistor 184 is substantially a linear function of the error voltage because of the linearity of the ramp voltage produced in ramp generator circuit 620. Therefore, there is substantially a linear relationship between the error voltage and the current shunted from the field winding 16.

More specifically, terminals 109, 111, 113 and 115 are connected to synchronizing pulse transformers 183, 185 and 187, as shown in FIG. 1 and hereinbefore described. Terminals 109, 111, 113 and 115 are also connected to the RC filtering circuit 600. The first of the three filtering and flip-flop networks is responsive to the output of synchronizing pulse transformer 183 and includes resistor 636 and capacitor 644, transistors 652 and 654, rectifiers or semiconductor diodes 662 and 664 and biasing resistors 656, 658, 666, 668, 672, and 674. The resistor 636 and capacitor 644 filter the pulses which are in synchronism with the output of vector summing transformer 66. These pulses are applied to the base electrodes of transistors 652 and 654. Transistors 652 and 654, which in this instance are of the NPN junction type having a base electrode b, emitter electrode a and collector electrode c are connected in the common emitter configuration. First, looking only at transistor 652, application of a signal to the base electrode b of transistor 652 will cause the transistor to saturate and act like a closed switch, grounding the output. If the input is grounded or unconnected, the transistor 652 loses its base drive and goes to the cut-off or non-conducting state, approaching an open switch. Some leakage current does flow at cutoff, however, and this is reduced by applying a small negative bias current from the negative terminal 160 through the resistor 656. By utilizing transistor 654 in conjunction with transistor 652 and connecting one input of each to the output of the other by lines 676 and 678, a bistable device known as a flip-flop circuit is created. Assume that in the initial state transistor 652 is non-conducting and transistor 654 is saturated. Application of a voltage pulse from the synchronizing pulse transformer 183 through the filter network 600 comprising resistor 636 and capacitor 644 will saturate transistor 652 and cut-off or cause transistor 654 to become non-conducting. The transistors 652 and 654 will stay in that condition even upon removal of the pulse, since the output of transistor 654 keeps transistor 652 saturated until a signal is applied to transistor 654. Therefore, a square wave is generated by the flip-flop circuit, comprising transistors 652 and 654, which is in synchronism with the output of vector summing transformer 66 and is applied to terminals 622 and 624. As hereinbefore mentioned, only the output from terminal 622 is used in this instance because the secondary of vector summing circuit 40 is connected in a three phase output rather than a six phase output.

Similarly, the filtering and flip-flop circuit responsive to the output of vector summing transformer 64 includes resistor 638 and capacitor 646, which form the filter network, transistors 680 and 682, rectifiers or semiconductor diodes 688 and 690, and biasing resistors 684, 686, 692, 694, 696 and 698. This circuit produces a square wave pulse synchronized with the output of vector summing transformer 64 and it is applied to terminals 626 and 628.

The third circuit, responsive to the output of vector summing transformer 62, is similar to the first two just described. Resistor 642 and capacitor 648 form the filter network, and the synchronous square wave generator or flip-flop circuit includes transistors 700 and 702, rectifier or semiconductor diodes 708 and 710, and biasing resistors 704, 706, 712, 714, 716 and 718. This circuit generates a square wave pulse synchronized with the output of vector summing transformer 62 and it is applied to terminals 632 and 634.

The synchronized square wave pulses produced by synchronizing circuit 610 are applied to firing circuits 730, 740 and 750, with the output from terminal 622 being applied to firing circuit 730 at input terminal 720; the output of terminal 626 being applied to firing circuit 740 at terminal 722; and the output of terminal 632 being applied to firing circuit 750 at terminal 724.

Firing circuit 730, in general, comprises a ramp generator 620, a differential amplifier 630, a flip-flop circuit 640 and an amplifier 650. In the operation of firing circuit 730, the synchronized square wave pulses from terminal 622 of synchronizing circuit 610 are applied to the input terminal 720. Ramp generator 620 includes transistor 760, having a base electrode b, collector electrode c and emitter electrode e, capacitor 612 and resistors762, 764, 766, and 768. During the half cycle of the reset square wave from circuit 610, the capacitor 612 is allowed to charge at substantially a linear rate. The collector resistance 762 of transistor 760 determines the time constant and rate of rise of the ramp voltage, while resistor 764 in series with capacitor 612 has relatively low resistance and limits the discharge current. Resistors 766 and 768 are biasing resistors. The ramp voltage produced by ramp generator circuit 620 is compared with the error voltage from the voltage regulating and current limiting circuits 50 and 55 using differential amplifier 630.

Differential amplifier 630 includes transistors 780 and 782, which may be of the NPN junction type having base electrode b, emitter electrode e and collector electrode 0, and resistors 784, 786, 788 and 790. The resistor 790 in the emitter circuit acts as a constant current source, with the current flowing through the transistor whose base is more positive. The other transistor will be cutoff or made non-conducting. The ramp voltage is applied to base electrode b of transistor 782 and the error voltage is applied to base electrode [2 of transistor 780. As the polarity of one base with respect to the other becomes more positive, there is a gradual transfer of current from one side to the other and it is during this portion of time that the circuit acts as a true differential amplifier. In this application, it is desirable for the transfer time to be as short as possible and therefore have the circuit function as a polarity detector with switching characteristics.

When the ramp voltage from ramp generator 760 applied to base b of transistor 7'82 exceeds the error voltage, the differential amplifier output voltage increases and is quickly driven to its maximum value. This rapid change in the differential amplifier output voltage is used to turn on a flip-flop circuit 640 which produces a steep wave front pulse with a width dependent upon the difference between the error voltage and ramp voltage. When the error voltage is small, the ramp voltage exceeds the error voltage earlier in the cycle than when the error voltage is large. Therefore, the differential amplifier circuit 630 produces an output earlier and flip-flop circuit 640 produces a pulse earlier, therefore generating a wider pulse when the error signal is small than it does when the error signal is large. Pip-flop circuit 640 includes transistors 800 and 882, which may be NPN junction transistors having a base electrode b, emitter electrode e and collector electrode 0, and biasing resistors 804, 805, 806, 808. 810, 812, 814 and 816.

The pulse generated by the flip-flop circuit 640 continues until the end of the positive half cycle. At this time, the reset voltage turns on and terminates the output from flip-flop circuit 640. The reset voltage is applied to the base of transistor 800 through resistor 814 from terminal 720. The reset voltage, at the same time causes transistor 760 to conduct, discharging capacitor 612. Thus, capacitor 612 is ready to build up or charge during the next positive cycle.

The square wave output pulse from flip-flop circuit 640 is applied to the amplifier circuit 650 and the amplified signal is applied to terminal 164 and hence to the gate electrode G of controlled rectifier 176. Amplifier 650 includes transistor 820, which may be an NPN junction type transistor having a base electrode [2, collector electrode and emitter electrode e, and resistors 822, 824 and 826. Amplifier 659 is a conventional emitter follower amplifier, taking the incoming signal from resistor 804 and producing an output at its emitter electrode e. Resistors 822, 824 and 826 are biasing resistors. Firing circuits 740 and 750 are similar in operation to firing circuit 730 just described and provides firing pulses at terminals 166 and 168 for controlled rectifiers 177 and 178.

In the operation of the firing and reset circuit 80,

pulses are produced in synchronizing pulse transformers 183, 185 and 187 in synchronism with the output of vector Summing circuit 40, as shown in FIG. 1. The pulses are filtered in filtering network 600 and form synchronized square waves in flip-flop circuits 610. These synchronized square waves control the timing of the ramp voltage generator 620 and flip-flop circuit 640. On one one-half cycle of the synchronized square wave input, capacitor 612 charges at substantially a linear rate, and on the next one-half cycle, capacitor 612 discharges and is then ready for the next half cycle. The ramp voltage produced in circuit 620 is applied to differential amplifier circuit 630 and it is compared with the error voltage produced by voltage regulator and current limiting circuits 50 and 55. As soon as the ramp voltage exceeds the error voltage, the dfierential amplifier circuit 630 produces an output which triggers or initiates the flip-flop circuit 640 has a width that varies with the error voltage. in circuit 650 and applied to the gate electrode G of controlled rectifier 176. The pulse generated by flip-flop circuit 640 has a width that varies with the error voltage. It starts during the positive half cycle when the ramp voltage exceeds the error voltage, therefore, the greater the error voltage, the narrower the width of the pulse and the smaller the error voltage, the greater the width of the pulse. The pulse ends at the termination of the half cycle, by virtue of the reset voltage being applied to the flip-flop circuit 640.

Therefore, when the error signal is large, the delay angle i increased because the ramp voltage exceeds the error voltage later in the half cycle. The square wave pulse applied to the gate electrode G of the controlled rectifiers is therefore narrow and the portion of current shunted through the shunting resistor 184 is relatively small, allowing a large excitation current to flow through field windings 16 of the generator 12. Conversely, if the error signal is small, the delay angle is increased. The square wave applied to the gate electrode of the controlled rectifier is therefore wider and the portion of current shunted around the field 1-5 is larger, reducing the excitation of current flowing through the field winding 16.

In summary, the function of firing circuit is to supply pulses to the gate electrodes G of controlled rectifiers 176, 177 and 178. The controlled rectifiers constitute the power amplifier element in the voltage regulating system and serve to control the amount of excitation energy delivered to the field winding 16 of the generator 12. The pulses delivered to the terminal 164, 166 and 168 of firing circuit 80 are synchronized with the altermating-current supply voltage supplied to the controlled rectifiers 176, 177 and 178 by vect-or summing circuit 40. The pulses cause the controlled rectifiers to fire or switch on once during each positive half cycle of the supply voltage at a delay angle. The delay angle is proportional to the amplified error signal obtained from the voltage regulator or current limiting circuits 59 and 55. Once the controlled rectifiers 176, 177 and 178 are turned on, they remain conductive until the end of the positive half cycle of the supply voltage. When the current through the controlled rectifiers 176, 177 and 178 drops to zero, or a value below their holding current, the gate electrode G regains control and blocks further flow of current until another signal is applied to said gate electrodes. The earlier in the cycle at which the controlled rectifiers are turned on, the greater the amount of energy shunted from the field circuit. Thus, the voltage regulator 50 controls generator excitation by varying the firing time of the controlled rectifiers by control of the firing circuit 88.

FIG. 9 illustrates another embodiment of the invention wherein a different means for shunting a portion of the field current is utilized. Like reference numerals in FIGS. 1 and 8 indicate like components in FIG. 1. The primary windings of vector summing transformers 62, 64 and 66 are connected as hereinbefore described relative .to FIG. 1. The secondary windings of said transformers, however, are connected in a six phase star arrangement, with each of the ix phases 9%, 902, 904, 906, 908 and 910 connected in a series circuit relationship with rectifier means 912, 914, 916, 918, 920 and 922, respectively. Shunting means comprising static switches and resistors shunt or by-pass each phase. The static switches are fired or made conductive in response to an error voltage produced by a voltage regulator circuit as hereinbefore described. FIG. 9 illustrates controlled rectifiers being used for the static switching means. However, power transistors or other static switches would be equally suitable.

More specifically, a resistor and static switch are connected in a series circuit relationship and the series circuit is connected in parallel with a secondary phase of vector summing transformers 62, 64 and 66. Controlled rectifier 924 and resistor 936 are connected in parallel with phase 910, controlled rectifier 926 and resistor 938 are connected in parallel with phase 908, controlled rectifier 928 and resistor 940 are connected in parallel with phase 906, controlled rectifier 930 and resistor 942 are connected in parallel with phase 984, controlled rectifier 932 and resistor 944 are connected in parallel with phase 902, and controlled rectifier 934 and resistor 946 are connected in parallel with phase 908. The same voltage regulator, current limiting circuit, preamplifier and bias circuit and reset and firing circuits may be used with the embodiment shown in FIG. 9, as hereinbefore described.

FIG. 10 illustrates still another embodiment of the invention utilizing electromechanical instead of static switching and shunting means. Electromechanical regulators of the direct or indirect acting rheostatic type are equally suitable. Like reference numerals in FIGS. 1

r and 10 refer to like components.

In the embodiment shown in FIG. 10, a resistor 996 with taps 998 is connected in parallel with the field winding 16 and is used to vary the amount of current shunted away from the field winding 16. Taps 998 are selected in response to an error signal from the regulator circuits. The primary windings of vector summing transformers 62, 64 and 66 are connected as hereinbefore described relative to FIG. 1.

In this instance, the secondary phases or winding 970, 972 and 974 are connected to three phase full wave rectifier 1000 at terminals 988, 990 and 992. Rectifiers 976, 978, 980, 982, 984 and 986 may be semi-conductor diodes.

A static excitation system constructed according to the teachings of this invention, whereby excitation current does not go through a regulating or control element, has greatly improved transient response characteristics. The ultimate in response characteristics is achieved because field forcing is instantaneously available. Transient response is only limited by the characteristics of the machine itself and not by the excitation system. If there is a requirement for maintaining a specified fault current for purposes of selective breaker tripping, it may be incorporated in the system herein disclosed without the disadvantages imposed by prior current limiting means. Prior art current limiting mean utilize saturation of cur rent transformers or reactors which produce voltage transients hamful to the field rectifiers. Consequently, costly excess rectifier capacity is required by the prior art systems. Current limiting by shunt control has none of these disadvantages since damaging transient voltages are not produced. Also, the system disclosed is insensitive to frequency, uses linear devices and standard components.

Since numerous changes may be made in the abovedescribed apparatu and different embodiments of the invention may be made without departing from the spirit thereof, it is intended that allmatter contained in the foregoing description or show-n in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

We claim as our invention:

1. A regulating system for a synchronous machine having an excitation field winding and output terminals, comprising:

vector summing transformer means including first and second primary windings and secondary windings; inductive reactor means connected in circuit relation with said output terminals and said first primary win-ding producing a first current responsive to the output voltage of said synchronous machine and lagging the line to neutral voltage of said synchronous machine by substantially ninety degrees;

said second primary winding of said vector summing transformer means producing a second current responsive to the line current of said synchronous machine;

said first and second currents combining vectorially in said vector summing transformer means and producing a third current in said secondary windings having a magnitude greater thanthe excitation current required by said field winding;

rectifier means connected in circuit relation with said secondary windings applying said third current .to said field winding;

means producing a reference voltage;

means producing an error signal responsive to the deviation of the output voltage of said dynamoelectric machine from said reference voltage; static switching means connected in parallel circuit relation with the secondary windings of said vector summing transformer means and responsive to said error signal for lay-passing a portion of said third current applied to said field winding so as to maintain the output of said dynamoelectric machine at a predetermined value;

means responsive to the output current of said synchronous machine for providing a limiting signal when a short circuit occurs at the output of said synchronous machine; I

and means for applying said limiting signal to said static switching means for by-passing a portion of said third current applied to said field winding so as to maintain the output current of said synchronous machine at a predetermined value.

2. A regulating system for a synchronous machine having an excitation winding and output terminals, comprising: 7

vector summing transformer means including first'and second primary windings and secondary windings; inductive reactor means connected in circuit relation with said output terminals and said primary winding producing a first current responsive to the output voltage of said synchronous machine and lagging the line to neutral voltage of said synchronous machine by substantially ninety degrees; the second primary winding of said vector summing transformer means producing a second current responsive to the line current of said synchronous machine; said first and second currents combining vectorially in said vector summing transformermeans and producing a third current in said secondary windings having a magnitude greater than required by said field winding; 1 rectifier means connected in circuit relation with said secondary windings and applying said third current to said field winding; means producing a reference voltage; means for producing an error signal responsive to the deviation of the output voltage of said dynamoelectric machine from said reference voltage; static switching means connected in arallel .circuit relation with the secondary windings of said vector summing transformer means and responsive to said error signal for by-passing a portion of said third current applied to said field winding so as to maintain the output of said dynamoelectric machine at a predetermined value; I means including current transformers and Zener diode means responsive to the output current of said synchronous machine for providing a limiting signal when a short circuit occurs at the output of said synchronous machine; 1

and means for applying said limiting signal to said static switching means for by-passing a portion of said third current applied'to said field winding so as to maintain the output of said synchronous machine to a predetermined value.

References Cited by the Examiner UNITED STATES PATENTS 2,289,059 7/1942 McNairy 322'25 X 2,332,942 10/1943 Shobert 322-25 X 3,027,509 3/1962 Lamaster 32225 3,030,568 4/1962 Soichi Oda et a]. 322-25 3,032,701 5/1962 Krausz 32225 3,098,959 7/1963 Rosenberry 318-18l 3,189,814 6/1965 Rosenberry et a1 322-25 3,217,229 11/1965 Ballard 32225 X MILTON O. HIRSHFIELD, Primary Examiner.

J. J. SWARTZ, Assistant Examiner. 

1. A REGULATING SYSTEM FOR A SYNCHRONOUS MACHINE HAVING AN EXCITATION FIELD WINDING AND OUTPUT TERMINALS, COMPRISING: VECTOR SUMMING TRANSFORMER MEANS INCLUDING FIRST AND SECOND PRIMARY WINDINGS AND SECONDARY WINDINGS; INDUCTIVE REACTOR MEANS CONNECTED IN CIRCUIT RELATION WITH SAID OUTPUT TERMINALS AND SAID FIRST PRIMARY WINDING PRODUCING A FIRST CURRENT RESPONSIVE TO THE OUTPUT VOLTAGE OF SAID SYNCHRONOUS MACHINE AND LAGGING THE LINE TO NEUTRAL VOLTAGE OF SAID SYNCHRONOUS MACHINE BY SUBSTANTIALLY NINETY DEGREES; SAID SECOND PRIMARY WINDING OF SAID VECTOR SUMMING TRANSFORMER MEANS PRODUCING A SECOND CURRENT RESPONSIVE TO THE LINE CURRENT OF SAID SYNCHRONOUS MACHINE; SAID FIRST AND SECOND CURRENTS COMBINING VECTORIALLY IN SAID VECTOR SUMMING TRANSFORMER MEANS AND PRODUCING A THIRD CURRENT IN SAID SECONDARY WINDINGS HAVING A MAGNITUDE GREATER THAN THE EXCITATION CURRENT REQUIRED BY SAID FIELD WINDING: RECTIFIER MEANS CONNECTED IN CIRCUIT RELATION WITH SAID SECONDARY WINDINGS APPLYING SAID THIRD CURRENT TO SAID FIELD WINDING; MEANS PRODUCING A REFERENCE VOLTAGE; MEANS PRODUCING AN ERROR SIGNAL RESPONSIVE TO THE DEVIATION OF THE OUTPUT VOLTAGE OF SAID DYNAMOELECTRIC MACHINE FROM SAID REFERENCE VOLTAGE; STATIC SWITCHING MEANS CONNECTED IN PARALLEL CIRCUIT RELATION WITH THE SECONDARY WINDINGS OF SAID VECTOR SUMMING TRANSFORMER MEANS AND RESPONSIVE TO SAID ERROR SIGNAL FOR BY-PASSING A PORTION OF SAID THIRD CURRENT APPLIED TO SAID FIELD WINDING SO AS TO MAINTAIN THE OUTPUT OF SAID DYNAMOELECTRIC MACHINE AT A PREDETERMINED VALUE; MEANS RESPONSIVE TO THE OUTPUT CURRENT OF SAID SYNCHRONOUS MACHINE FOR PROVIDING A LIMITING SIGNAL WHEN A SHORT CIRCUIT OCCURS AT THE OUTPUT OF SAID SYNCHRONOUS MACHINE; AND MEANS FOR APPLYING SAID LIMITING SIGNAL TO SAID STATIC SWITCHING MEANS FOR BY-PASSING A PORTION OF SAID THIRD CURRENT APPLIED TO SAID FIELD WINDING SO AS TO MAINTAIN THE OUTPUT CURRENT OF SAID SYNCHRONOUS MACHINE AT A PRETERMINED VALUE. 